Date: Mon, 10 Mar 2003 13:26:17 -0500 (EST) From: John Baldwin <jhb@FreeBSD.org> To: Poul-Henning Kamp <phk@phk.freebsd.dk> Cc: src-committers@FreeBSD.org, cvs-src@FreeBSD.org, Nate Lawson <nate@root.org> Subject: Re: cvs commit: src/sys/sys devicestat.h Message-ID: <XFMail.20030310132617.jhb@FreeBSD.org> In-Reply-To: <2986.1047320558@critter.freebsd.dk>
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On 10-Mar-2003 Poul-Henning Kamp wrote:
> In message <Pine.BSF.4.21.0303101009420.93727-100000@root.org>, Nate Lawson wri
> tes:
>
> Doesn't work:
>
> CPU0 (kernel) CPU1 (userland)
>
> increment seq
> start copy
> fiddle data
> end copy
> (copy is inconsistent)
>
> You need two sequence fields:
>
> CPU0 (kernel) CPU1 (userland)
>
> increment seq0
> start copy
> fiddle data
> end copy
> increment seq1
> (seq0 != seq1 indicates inconcistent data)
>
> Notice that to make it absolutely water-tight, the seq fields must
> be updated with appropriate write barriers and the order of the
> fields must be the opposite as the direction the copy is performed in.
Correct. If you use:
atomic_add_acq_int(&seq0, 1);
... all the updates ...
atomic_add_rel_int(&seq1, 1);
then you should have sufficient membars.
--
John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve!" - http://www.FreeBSD.org/
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